We have studied the biosynthesis of glycoprotein hormones in various normal and neoplastic cells derived from the pituitary and placenta. Cell-free translation of pituitary messenger RNA has demonstrated separate synthesis of a precursor form of TSH-alpha subunit (pre-alpha) and of TSH-beta (pre-beta). The pre-alphas of three species have been microsequenced and found to have initiating methionine residues as well as complete homology in the position of leucines. Studies in intact thyrotrophs have demonstrated processing of pre-alpha initially to a form cleaved of the pre-piece and core-glycosylated, followed by addition of outer sugars necessary for combination with the limiting beta subunit. Biosynthesis of TSH is stimulated by thyrotropin releasing factor and potassium and inhibited by thyroid hormones and somatostatin. Study of chorionic gonadotropin synthesis by choriocarcinoma cells has shown that only a small form of HCG-alpha acumulates in the cells while a processed large alpha form is selectively secreted along with complete HCG. Similar large forms of alpha are found in human sera and can be converted back to smaller forms by the selective removal of carbohydrate with mixed glycosidases.